Radio frequency attenuator



y 8, 1969 v. E. RIMSHA 3,454,907

RADIO FREQUENCY ATTENUATOR Filed Aug. 1, 1966 Victor E. Rimsha,

INVENTOR.

m BY M 0. MW

M w UM United States Patent 3,454,907 RADIO FREQUENCY ATTENUATOR Victor E. Rimsha, Santa Ana, Califi, assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Army Filed Aug. 1, 1966, Ser. No. 569,526 Int. Cl. H01h 7/14 US. Cl. 333-79 3 Claims ABSTRACT OF THE DISCLOSURE A radio frequency attenuator in the form of an electrical conductor for attenuating radio frequency signals. The electrical conductor is made of copper clad by a nickel cylindrical conductor and has a high magnetic permeability alloy wire wound helically around the conductor. The high magnetic permeability alloy Wire is heat fused to the electrical conductor and is insulated by an insulated cover.

This invention relates to radio frequency attenuators and more specifically to electrical conductors for attenuating radio frequencies.

When an electroexplosive device (EED) such as a detonator, squib, igniter, etc. is placed in an electromagnetic field, such as created by radar, communication or telemetry transmitters, or by a nuclear reaction, a hazard arises. The BBB and its associated circuitry may couple energy from the electromagnetic radiation, and an inadvertent initiation of the EED may occur. This hazard arises whenever a missile or a space vehicle is in the vicinity of radio transmitters, or nuclear reaction. This situation exists when the vehicle is on the launch pad. immediately after take-off when it is in the field of local transmitters, and at any time after launch if it is in an electromagnetic radiation field for radio or nuclear sources.

Many devices or procedures have been considered and/ or used to provide partial protection such as shielding, isolating, and balancing wiring, decreased EED sensitivity, filter networks, carbonyl iron ferrite attenuators, exploding bridgewire and band pass transformers. All of the above devices fall far short for providing a solution to the problem.

In view of the above deficiencies, it is, therefore, an object of this invention to provide a conductor of high magnetic permeability alloy With high skin effect resistance that provides adequate attenuation irrespective of the frequencies encountered.

Further, it is an object of this invention to provide a conductor having attenuation of alternating currents at all radio frequencies.

Another object of this invention is to provide a radio frequency attenuator which eliminates notches at sneak frequencies.

Still further, it is an object of thi invention to provide a radio frequency attenuator which is cheaper and easier to manufacture.

These and other objects and the attendant advantages, features, and uses will become apparent to those skilled in the art as the description proceeds when considered in conjunction with the single figure of a radio frequency attenuating conductor according to the present invention.

Referring now to the figure, the conductor is shown and consists of an inner solid conductor 5 made of copper being clad with a nickel cylindrical conductor 7. About this center core consisting of the copper conductor 5 and nickel clad 7 is wound, in a spiral manner, a wire 9 made of a metal having a high mu characteristic such a Permalloy. Wire 9 is heat fused to conductor 7 to insure proper condution producing a high skin effect of RF frequencies. The entire conductor is then insulated by an insulating cover 11 in a conventional manner such as being inserted into Teflon spaghetti tubing.

The configuration can be varied in size and alloy composition. The DC resistance of the conductor is controlled primarily by the inner core, the copper plus nickel. Enlarging the copper diameter provides a lower DC resistance, which is desirable. If the inner conductor 5 is of iron or similar magnetic material, DC resistance is increased, but the RF performance is improved.

The nickel cladding over the copper provides strength during the heat treat of the conductor fusing wire 9 to conductor 7, and simultaneously improves the RF performance. Other low mu materials may be used for the inner core. The compromise is between strength at heat treat temperature, corrosion resistance, and mu parameter. The outer layer of the conductor, wire 9, has high mu characteristics and provides the skin resistance at high frequencies, which accomplishes the attenuation function. The thickness of this layer can be varied, by using larger or smaller wire to wrap around the inner core. This variation becomes a trade-off between RF performance and size of conductor, and it has been found that the relationship of performance to size is non-linear, Further, it should be pointed out that a wire with low resistance core, iron, with permalloy clad over it by wire draw process, does not produce the high skin resistance. However, when a high mu metal is wound in a helix over the composite nickel/ copper core and fused to the core by means of electrical heating, the high skin resistance effect is obtained. The reason for which will be explained presently.

IN OPERATION When a DC current is passed through the conductor, the current is uniformly distributed through the cross section of the inner core (conductors 5 and 7). The resistance of the conductor to current flow is, by definition, the true or ohmic resistance. From this resistance, the resistivity of the particular metal or alloy per unit volume is computed. Such values are listed in handbooks.

When alternating currents are induced or applied to the conductor, there is a greater current density near the surface of the conductor. This is known as skin effect. As frequency increases, the current is crowded more and more to the surface of the conductor, until at high frequencies, nearly all the current is in the very thin film at the surface, i.e., in the skin of the conductor. The effective resistance increases with frequency. Since the spiral conductor 9 is heat fused to conductor 7, it acts as the outermost part of the entire conductor, and being of a high mu material, it provides a high resistance to AC currents which is the needed requirement for protection of an EED while DC currents are easily passed through conductors 5 and 7.

This cable has been tested extensively, and a five foot section has the following ohmic resistances:

Frequency: Ohmic resistance, ohms DC 1.5 100.0 kc -150 In addition to increased attenuation of RF frequencies, there are no sneak frequency notches; thus, eliminating the need for a filter network, it is cheaper to manufacture and it is more reliable.

This cable is easily adaptable to winding coils for detonators and the like where there is a danger of premature ignition due to induced AC currents.

While in accordance with the provisions of the statutes, the best form of the invention now known has been illustrated, it will be apparent to those skilled in the art that changes may be made in the form of the invention disclosed without departing from the spirit of the invention.

Accordingly, it is desired that the scope of this invention be limited only by the appended claims.

What is claimed is: r

1. A radio frequency attenuator conductor comprising:

Permalloy having a high mu factor for greater RF attenuation.

3. A radio frequency attenuator conductor as set forth in claim 2, wherein said insulating means comprises a Teflon an inner core conductor of a metal having a low mu factor, 5 spaghetti tubing placed over said helical wound outer consaid inner core conductor comprising a solid copper conductor and a cylindrical nickel cladding surrounding said copper conductor; a helical wound outer conductor of a metal having a high mu factor for attenuating AC currents forced into conduction in said helical conductor due to skin effect, said helical conductor being heat fused to said inner core conductor to insure transfer of AC currents to said helical conductor, said cylindrical nickel cladding surrounding said copper conductor for providing strength to said conductor during heat fusion of said heli- 5 cal conductor to said inner core; and an insulating means surrounding said outer conductor.

2. A radio frequency attenuator conductor as set forth in claim 1, wherein said helical conductor is made of duuctor.

References Cited UNITED STATES PATENTS 3,219,951 11/1965 Clark 333-79 3,191,132 6/1965 Mayer 33379 2,756,394 7/1956 Sieven 33331 3,329,911 7/1967 Schlicke 33379 HERMANN KARL SAALBACH, Primary Examiner.

C. BARAFF, Assistant Examiner.

US. Cl. X.R. 

